In Watchtower theology Jesus is mediator of the New Covenant. That much is scriptural. They limit the new covenant participants to God and the 144,000. A covenant is a contractual agreement. The new covenant is described in Jeremiah. The new covenant organizes spiritual Israel as a nation. A mediator is different from a propitiator. In Watchtower theology Jesus is the propitiator (peace maker, reconciler) for all. He is the mediator between God and the Bride of Christ in the New Covenant relationship.
If one believes that only 144K are part of that covenant, then one must believe that Jesus' mediatorship concerns only them and God, the parties to the covenant. There is a distinct difference between a covenant instituting sacrifice and a propitiating (sin atoning) sacrifice. In Christian theology, Jesus' one sacrifice filled both functions. There is Old Testament precedence for that.
The Watchtower does not teach that Christ's sacrifice does not cover all. While their teaching on mediatorship may be flawed, it is consistently misrepresented. They do not teach that all Christians are not covered by Christ's atoning sacrifice. Witness soteriology is unnecessarily convoluted.
Personally, I’m inclined to see the number 144,000 as symbolic, though I admit there is some merit in the Watchtower argument that it is a literal number. The issue rests with God, and, unlike the Watchtower that brooks no contrary view, I’m willing to let it rest with him. I see all the issues as to the timing of ultimate salvation as a replay of 19th Century discussions that went no where then and are meaningless now.
The problem isn’t that they see Jesus' mediatorship as confined to a small number. They see his propitiating sacrifice as applying to all, and that is sound Christian doctrine. The problem rests in their tendency to make basic doctrine as hard to understand as is possible. And they express what are no more than personal beliefs as if they were the inspired word of God.